Angular providers: Use existing or create one if not provided

Question

In Angular 2+, how can I provide a new instance of an Injectable only if there's no instance existing?

I searched and found none so I tried useFactory. This is my code so far:

{
  provide: ColorsService,
  useFactory: (colorService: ColorsService, colors: string[]) => {
    if (colorService) return colorService;
    return new ColorsService(colors);
  },
  deps: [ColorsService, MY_COLORS],
},

Besides the fact that I'm injecting the MY_COLORS token manually, I get a weird error:

Error: Circular dep for ColorsService

Answer 1

So with the help of @eko I found this solution:

@Component({
  ...
  providers: [ColorsService],
})
export class ChildComponent {
  colorService: ColorsService;
  constructor(
    @Self() currentColorService: ColorsService,
    @Optional() @SkipSelf() parentColorService?: ColorsService,
  ) {
    this.colorService = parentColorService ?? currentColorService;
  }
}

In this example, the parentColorService is the service that might be provided by a parent component. If it's not provided, I'll use the currentColorSerivce which is instantiated in this component (it's instantiated anyway which may not be a good thing).